Feature
by Satoshi Ochi, Kazuhiko Kagawa, Mitsubishi Electric Corporation
and Glenn A. Calhoon, Kevin Beamer, Mitsubishi Electric Power Products Inc.
Vacuum Circuit Breaker Technology
Vacuum Interrupters – How They Work
T
he continuing use of vacuum interrupter technology
in the power distribution market as replacements
for oil circuit breakers has exposed this technology
to people who have not been previously exposed to it. This
article covers the features of the vacuum interrupter and how
they correspond to the ability of the interrupter to perform
according to its ratings.
Vacuum interruption technology has been used for
many years and has proven itself to be a reliable means for
interrupting fault currents in distribution switchgear. The
application of vacuum circuit breakers as replacements for
oil circuit breakers offers many advantages to the user in
areas such as maintenance, performance, and environmental
Bushing
Bushing current
transformer
Mechanism
access door
High-voltage
compartment
High-voltage
access door
Breaker module
Mechanism
access door
concerns. From a maintenance point of view, with a vacuum
breaker there is no oil handling required with the associated
clean up as well as long contact life and lower mechanism
operating forces due to the small size and stroke required
by a vacuum interrupter. A typical vacuum interrupter as
applied in a distribution breaker will easily handle 10,000
operations at rated continuous current and more than 20 full
fault operations without the need for contact replacement.
Also, as a result of the lower mechanical requirements the
mechanisms in today’s vacuum breakers are much simpler
in design with fewer moving parts and lighter loads being
applied.
1. Breaker Overview
The structure of an outdoor vacuum circuit breaker (VCB)
is shown in Fig.1. The three-phase ganged interrupter assembly is mounted in a self-contained breaker module
containing the operating mechanism, auxiliary switch,
mechanical linkage, and vacuum interrupter (VI)
mounted in its own support insulator. The mechanical
linkage and supporting framework are isolated from
the high voltage interrupter by an insulated drive rod
and the interrupter support insulators.
2. Features of Vacuum Interrupters
Vacuum interrupter
Low-voltage
compartment
2.1 The Property of Vacuum
Low-voltage
access door
Integral internal
ground pad,
typ 2 corners
Conduit entrance
Ground pad
typ opposite corner
Figure 1 — Structure of an Outdoor VCB (17.5 kV-25 kA)
The dielectric strength of a vacuum is superior to
other dielectric mediums such as air and SF6 gas as
shown in Fig. 2. Due to high dielectric strength of a
vacuum environment, it is possible to reduce the arcing
time of the interrupter as the dielectric capabilities of
a vacuum can better withstand the generated transient
recovery voltage (TRV). The net result of this is the
This article was developed from the 2006 Doble Client Conference Paper “Vacuum Circuit Breaker Technology: Vacuum Interrupters
– How They Work” by Satoshi Ochi, Kazuhiko Kagawa, Glenn A. Calhoon, and Kevin Beamer.
Published with permission of Doble Engineering Company.
www.netaworld.org Summer 2010 NETA WORLD
1
Vacuum
SF6 (0.1MPa)
Air (0.1MPa)
the magnetic force generated by the current flow, this will
cause the arc to be forced from the point of origin across the
spiral petal to the outer diameter of the contact. Once the
arc is on the outer edge of the contact, the magnetic force
along the outer diameter will cause the arc to rotate around
the outside diameter of the contact until it is extinguished.
The AMF contact structure will generate an axial magnetic
field caused by the current flow through the contacts which
in turn will create a diffuse arc (low energy arc) over the
entire contact area dispersing the arc energy over the contact
surface area. The diameter of spiral contacts can be smaller
than that of the AMF contact structure for the same fault
current rating due to its ability to rapidly move and control
the arc energy. In comparison the AMF contacts are superior
for applications which require long arcing times and require
a high number of full fault interruptions.
Figure 2 — Dielectric Strength Properties of Vacuum
minimum arcing time of a 15 kV vacuum circuit breaker
is approximately 1 to 2 ms. The higher dielectric capability
of the vacuum will yield a lower arc voltage and therefore
reduce the amount of arc energy to be dissipated across the
contacts in comparison with other interrupting mediums.
This results in vacuum interrupters being able to break
large fault currents with a short interrupting time and with
minimal contact erosion. These properties allow vacuum
interrupters to have a very compact design.
2.2 Basic Structure of Vacuum Interrupters
The basic structure of a vacuum interrupter is shown in
Fig. 3. The main contacts are installed in a ceramic insulator
-5
in which high vacuum approximately 10 Pa is maintained.
The movable terminal is connected to the breaker mechanism through an insulated drive rod which will open and
close the interrupter contacts. A stainless steel bellows is
used to allow contact travel and still maintain the integrity of
the vacuum in the interrupter. The contact material strongly
affects the interrupting capability, service life, and reliability
of the vacuum interrupter. There are three types of contact
designs used in vacuum interrupters, and they are applied
based on their application as shown in Table 1. Generally,
in applications with fault currents below approximately 20
kA, a flat butt type of contact design is sufficient. In applications where fault currents can exceed 20 kA, the design
limitation of the contact will manifest itself as a localized
high energy arc which results in large amounts of metal
vapor being placed in the contact zone and thereby limiting
the fault rating of the interrupter. In order to improve the
interrupter’s fault capacity either the spiral or axial magnetic
field (AMF) contact design can be applied. These contacts
are used in higher fault ratings as they are able to utilize the
force created by the magnetic fields associated with the fault
current. This enables the interrupter to effectively control
the generated arc and therefore create a more efficient interrupter design. The spiral contacts will generate a magnetic
field caused by the current flow through the contact. Due to
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NETA WORLD Summer 2010
Figure 3 — Basic Structure of a Vacuum Interrupter
Table 1
Typical Contact Structures
Contact
Structure
Flat Contact
Spiral Contact
Axial Magnetic
Field Contact
VCB (General)
LBS Contactor
VCB
(Large capactiry)
VCB
(More interruption)
VCB
(Low surge)
Cu-Cr alloy
Ag-WC alloy
Cu-W alloy
Cu-Cr alloy
Cu-Cr alloy
Ag-WC alloy
Figure
Application
Materials
www.netaworld.org
thus metal vapor in the contact area can be controlled. This will result in greater interrupting ratings for
the contacts and longer contact life.
2.3 Arc Control During Interruption
By usingArc
a high-speed video camera and
a specially
Arc
Arc Appears
between
Arc
between
Arc appears
appears between
contacts
adapted vacuum chamber, the motion of the
arc movecontacts
contacts
11ms
ms
1.5ms
ment can be observed.
Fig. 4 shows the movement of the
arc for a spiral contact configuration during fault current
The arc starts to move
Arc starts
to diffuse
by by
The
arc starts
to diffuse
interruption. At contact part plus 1.5 ms, the
arc has been
by Magnetic drive
Axial magnetic
Magnetic Field
axial
field
established between the contacts. At contact part plus 2 ms
33ms
ms
2.0ms
the arc is beginning
to rotate around the outer edge of the
contacts as a result of the magnetic forces that are being
Arc
and
Arc spreads
spreads and
Rotation
of Arcthe
generated from the flow of the fault current
through
interruption
is completing
completing
interruption is
contacts. This movement
of the arc around the outer diam2.5ms
55ms
ms
eter of the contact surface provides the greatest amount of
surface
the Control
arc to travel
overInterruption
thus providing aFixed
larger
area
contact
Fixed contact
2.3forArc
During
to distribute the heat generated from the arc. The net result
of this
of the arc video
is less localized
overheating
of adapted vacuum chamber the motion Movable
Bymovement
using high-speed
camera and
a specially
of the contact
arc
Movable
contact
the contact
surfaces
and
therefore
less
metal
vapor
being
movements can be observed. Fig. 4 shows the movement of the arc for a spiral contact configuration
introduced
area between
the contact
surfaces.part
This + 1.5ms the arc has been established between the
duringinto
faultthecurrent
interruption.
At contact
greatly
improves
the
interrupting
capability
of
the
intercontacts. At contact part + 2ms the arc is beginning to rotate around the outer edge of the contacts as a
The Arc
of spiralthe
contacts
The Arc motion of AMF contacts
rupterresult
and also
results
in motion
less erosion
contact
of the
magnetic
forces of
that are
beingsurfaces,
generated from theFigure
flow 5of— the
fault
current
through
the
The Arc
Motion
of AMF
Contacts
improving
the life
the interrupter.
contacts.
Thisofmovement
of the arc around the outer diameter of the contact surface provides the greatest
Fig.
5 shows
the arcing
thatarc
takes
place over
in anthus
AMF
amount
of surface
for the
to travel
providing a larger area to distribute
the heat
FIGURE
4
FIGURE
5 generated
contact
configuration
during
fault
interruption.
At
contact
from the arc. The net result of this movement of the arc results in less localized overheating of the
2.4
Short Circuit
part plus
1 ms,
the arcand
is beginning
dispersed
as being
a
contact
surfaces
therefore to
lessbemetal
vapor
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area Interruption
between the contact
resultsurfaces
of the axial
magnetic
fields generated
by the contact
6 shows a typical
oscillograph
during
short-circuit
thisShort
greatly
improves
the
interrupting
capability ofFig.
the interrupter
and also
results in less
erosion
2.3
Circuit
Fault
Interruption
geometry.
This
dispersal
will
generate
numerous
low
energy
current
interruption.
When
the
contacts
separate
during
of the contact surfaces improving the life of the interrupter.
arcs (macroscopically one diffused arc) which will spread
fault
interruption
an
arc
appears
between
the
contacts
and
Fig.6
shows
a typical
oscillograph
during
short
circuit current interruption. When the contacts separate
out across
the
entire
contact
surface
by
contact
part
plus
3
is
maintained
until
the
next
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zero
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an arc
contacts
and is maintained
the next current
ms. As
the contacts
to separate,
the appears
density
of the the
tered.
At the
same
time,
an the
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1ms
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so
that
at
contact
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will
by the contact geometry. This dispersal will generate rdnumerous low energy arcs (macroscopically one
plus 5diffused
ms interruption
will occur
when
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© 2006
Doble
Engineering
Company-73
Annual International
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appear
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is encountered.
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the
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contacts continue to separate the density of the diffuse arc will continue to decrease so that at contact part
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into numerous low energy arcs spread across the entire contact surface the amount of contact erosionofand
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thus metal vapor in the contact area can be controlled. This will result in greater interrupting ratings for
interrupting
ratings
the contacts
at the next current zero crossing. This condition will occur
the contacts
andforlonger
contactand
life.longer contact life.
when the time to the first zero crossing event is less than
the designed minimum arcing time of the interrupter.
Arc
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The Arc motion of spiral contacts
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6—
Oscillograph
The Arc motion
AMF
contactsDuring the Interruption
Figure 4 — The Arc Motion of Spiral Contacts
FIGURE 4
www.netaworld.org 2.3 Short Circuit Fault Interruption
FIGURE 5
Summer 2010 NETA WORLD
3
3. Design Test
The capability of a medium-voltage circuit breaker is
verified by design tests specified in the IEEE/ANSI standards, C37 series. In the design tests, a series of capabilities
(dielectric, current carrying, short-circuit current interruption, and other current switching) are tested to meet the
required ratings. The following items, based on the design
test requirements, need to be taken into consideration
when demonstrating the circuit breaker’s capability and
application.
3.1 Asymmetrical Short-Circuit Current
Interruption and the Effect of Various X/R
Ratios
3.1.1 The rated interrupting time of a circuit breaker is
defined as the maximum permissible time interval
between the energization of the trip circuit (at rated
control voltage), and the interruption of the current
in the main circuit in all three phases. The standards
allow for ½ cycle of relay time prior to the trip circuit energizing. This ½ cycle of relaying time is not
allowed to be counted as part of the interrupting
time of the circuit breaker. A circuit breaker rated
for three cycles must completely interrupt all current
flowing in the main circuit by the time three cycles
of 60 Hz current has passed (50 ms) after the trip
circuit is energized. If the current continues to flow
beyond the 50 ms, the breaker cannot be rated as a
three-cycle breaker. A five-cycle breaker is one that
interrupts the current between three and five cycles
(50-83 ms) after the trip circuit is energized. If there
is any current flow after 83 ms, the breaker cannot be
rated as a five-cycle breaker. The interrupter must be
able to interrupt current flow on the first or second
current zero after contact part. See Fig. 7.
Figure 7 —Three-Phase Current Interruption for Three-Cycle CB
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NETA WORLD Summer 2010
3.1.2. A typical short-circuit asymmetrical wave is shown in
Fig. 8. For a three-cycle circuit breaker, it is necessary
to complete the interruption where the interrupting
time is equal to or less than three cycles. Moreover,
the test procedure in the IEEE standards requires
demonstrating that an interrupter can reliably
interrupt current under the most severe switching
conditions with the maximum arcing energy (longest
arcing time and a major loop of asymmetrical current). Symmetrical interruptions must be performed
to meet the ANSI/IEEE standards. The test duties
included in Table 1 of IEEE C37.09-1999 demonstrate the interrupter’s capability to interrupt current
flow, both symmetrical and asymmetrical.
Figure 8 — Short Circuit Asymmetric Current (for 25 kA)
3.1.3 The current peak value and major loop duration
are controlled by the value of X/R. The X/R ratio
is the ratio of the main circuit’s inductance divided
by the resistance. The higher the ratio is, the slower
the decay of the dc component of the asymmetrical
fault current. Fig. 8 shows the decay characteristic
of an asymmetrical current. Notice the peak current
decreasing with time. A standard value of X/R is
given as 17 at 60 Hz in IEEE Std C37.09-1999.
However, the purchaser of the breaker needs to evaluate his system and verify that the X/R of the system
is less than 17. When the circuit has an inherently
high X/R, the interrupting time must still meet
the interruption rating of the breaker (three or five
cycles); however, the arcing time will increase due to
a larger major current loop. The major loops will not
only have an increased time between current zeros, it
will also have a higher peak current due to a higher
X/R ratio. Very high X/R ratios may not even have
a current zero for the first couple of cycles. For example, if you compared an X/R=30 with an X/R=17
(see Fig. 8), the arcing time increases by a factor of
1.1, and the current peak value increases by a factor
of 1.4.
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3.1.4 Symmetrical current interruptions must also be performed according to IEEE C37.09-1999, Table 1.
The symmetrical interruptions are required for design
testing and to verify that the contact serviceability
as stated in IEEE C37.04-1999 section 5.8.2.5 is
achieved. The standards require a minimum of 800
percent of the required asymmetrical interrupting
capability of the circuit breaker be accumulated on
a single interrupter. Contact serviceability is the
amount of contact erosion that the interrupter must
be able to withstand and still be able to perform a
successful interruption.
3.1.5 There are several design features that cause a vacuum
interrupter to be superior to other types of interrupters for this application. The AMF style of contact
inside the interrupter allows for a longer arcing time
due to the low current density of the diffused arc. This
reduced energy will keep the surface of the contacts
from overheating and thus reduce the possibility of
restrikes. The reduction of restrikes is due to reduced
hot spots and reduced metal vapor in the contact gap.
TRV (kV)
3.2 Transient Recovery Voltage
For circuit breakers rated below 100 kV, in IEEE Std
C37.04-1999, the rated TRV is represented by a 1-cosine
wave, while on the other hand, in IEC 62271-100, 2003
the TRV is represented by an exponential-cosine wave as
shown in Table 2 and Fig. 9. These ratings are for shortcircuit tests, duty 4 and duty 5 in IEEE C37.09 and for
terminal fault test, duty T100a, in IEC. It is worth noting
that in the IEEE standards the TRV specifications are
different for outdoor and indoor applications. The outdoor
specification is more severe than the indoor. Breakers tested
and rated for indoor use, may not be suitable for outdoor
applications. Also, the differences between the IEC and
the IEEE TRV requirements are significant enough that
breakers tested to the IEC standard may not pass testing
for IEEE. The steeper slope of the recovery voltage will
change the requirements of the contact design to achieve a
greater rate of dielectric recovery inside the interrupter. The
capability of the interrupter to successfully interrupt this
high rate of rise of recovery voltages (RRRV) is dependent
upon the internal geometry of the interrupter. The type of
contact and the surrounding dielectric fields produced by
the internal shield all have a large impact on the capability
of the interrupter. The AMF style of contact is well suited
for this purpose due to reduced hot spots and metal vapor
in the contact gap. The smooth contact surface allows for
a very rapid dielectric recovery of the gap which allows
the vacuum interrupter to withstand the TRV better than
other types of medium. The rapid recovery allows for small
contact gaps, which translates to a very short stroke of the
interrupter and operating mechanisms.
3.3 Continuous Current Carrying Capability of
Vacuum Interrupters
3.3.1 The rated continuous current of a breaker is determined by the current carrying capability without
exceeding the temperature limitations as listed in
Table 2 of IEEE C37.04-1999. This temperature
limit takes into consideration the types of materials,
plating, and atmosphere that surround the point
where the temperature is being measured.
Time (µs)
Figure 9 — TRV Waveform for 15-17.5 kV Breaker
The TRV characteristics depend on the system in which a
circuit breaker is installed. The TRV is controlled mainly by
the amount of inductance and/or capacitance on each side
of the breaker. However, to provide a general application
guide, the TRV values under most system circuit conditions
are specified in the following standards.
Rating of TRV for 15-17.5 kV Breaker
IEEE Indoor
15 kV
Contact
Silver-coated in air
Total temperature (°C)
Temperature rsie at
ambient ª40°C)
105
65
115
75
IEC
17.5 kV
Peak (kV)
29
28
30
Rise (kV/µs)
0.81
0.37
0.42
www.netaworld.org Nature of the part and material
Connections,
bolted or the
equivalent
Table 2
IEEE Outdoor
15.5 kV
Table 3
Example of the Limits of Total Temperature and Temperature Rise
Summer 2010 NETA WORLD
5
temperature rise associated with the
higher resistance. The spiral or butt
Example of Typical Measurement Points and the Temperature Limitation (°C)
type contact is generally better suited
for high continuous currents than the
Measurement Incoming Bus Top of
Upper Bus
Top of VI
Moving
Lower Bus
AMF style of contact. Consideration
point
Bushing
Stem of VI
must also be given to the location
Temperature
65° Tin
75° Silver
75° Silver
75° Silver
75° Silver
± 5° from
where the circuit breaker will be inrise limit
coated
coated
coated
coated
top of busing coated
stalled. If the installation site has an
temp during
ambient temperature rating of 50°C,
testing
as compared to the 40°C used in
the IEEE standards, then the total
temperature rise must be decreased accordingly (by
The most common spot in a VCB where temperature
10°C in this case). The overall performance of the
rise is a concern is the moving stem of the vacuum
interrupter needs to be considered to determine the
interrupter. It is necessary to monitor the temperature
best contact design, material, and size for any given
very closely at this critical connection point where the
application. To ensure proper application of the circurrent path must allow for the motion of conductors.
cuit breaker, the purchaser and manufacturer should
A typical temperature rise graph is shown in Fig. 10.
review the test reports.
In a medium-voltage VCB, the thermal time constant
is usually small. If a VCB is required to carry overload
3.4 Dielectric Capabilities of Vacuum
current for several hours, the temperature rise on a
Interrupters.
VCB will approach the saturation values. This tem3.4.1 Vacuum interrupters are well suited to withstand
perature may be over the thermal limits of materials.
voltage surges due to the rapid recovery of the dielecWhether or not the breaker will exceed any critical
tric strength of the contact gap caused by the high
temperature limits will need to be evaluated carelevel vacuum. Other types of interrupter medium
fully. The IEEE standards cover standard overload
take much longer to recover due to the need to rerequirements; refer to IEEE C37.10-1999 section
move hot gas (in SF6 for example) from the gap. The
5.4.4. If the breaker will be required to exceed these
smooth contact surfaces and surrounding geometry
limits, the purchaser and manufacturer will need to
(primarily the internal shield), as well as the high
review the continuous current test data to verify the
level of vacuum, all contribute to the interrupter’s
breaker’s capability for higher overload ratings.
voltage withstand capability.
Table 4
3.4.2 Lightning Impulse Withstand Voltage (BIL)
Temperature rise (K)
Time (min)
Figure 10 — Example of Temperature Rise Time Charts
3.3.2 The size and design of the main contacts and conductors contribute to the continuous current rating
of the interrupter. The larger the contact diameter,
the better the current carrying capability. The contact
design and material also play a part in the continuous
current rating. The different contact materials may
have different resistance values. A higher resistance
value may limit the amount of current due to the
6
NETA WORLD Summer 2010
Vacuum is very reliable during lightning impulse
withstand voltage. This is demonstrated by testing
performed per section 4.4. of IEEE C37.09-1999.
The flat design of contacts and the voltage profiling of the surrounding components (for example,
the internal shield, bellows, moving contact stem,
etc.) make this a very impulse friendly interrupter.
Another factor that allows the small contact gap to
withstand voltage surges is the high level of vacuum
inside the interrupter. A high level vacuum eliminates
all foreign particles within the interrupter. This results
in having no impurities and a very low molecular
count across the contact gap. The absence of foreign
particles and molecules will not allow the voltage to
jump across the internal gap.
There are several ways of improving the external
dielectric capabilities of a vacuum interrupter. Many
manufacturers will use various types of minor modifications to improve the dielectric capabilities, or
margins, within a particular interrupter design. Some
examples are the use of an externally contoured, or
shedded, porcelain body to increase the creepage
distance. The increase in creepage distance allows for
more margin in a high contamination environment.
www.netaworld.org
Another example would be to use a heat shrink material on the ends to increase creep and strike. The use
a potting material to completely cover the outside of
the interrupter to increase the dielectric strength has
also been used for increased margins or capabilities
of a given interrupter design.
Vacuum interrupters are ideal for use in low-, medium-,
and even the lower end of high-voltage applications. Vacuum will eventually be used in high-voltage applications
as technology keeps improving the capability of vacuum
interrupters. The internal design of the vacuum interrupter,
especially the main contacts, greatly impacts the capability of
the interrupter. There are different types of contact designs
that will utilize different materials to achieve different ratings and capabilities. The internal design of the interrupter
will depend on the rating of the circuit breaker where the
interrupter will be installed. Some of the items to consider
on the internal design are not only the contact shape but also
the internal shield shape, the moving stem shape, blending
radii, and even the mounting configuration of the vacuum
interrupter. Whether the breaker will be primarily used for
capacitor switching duty, transformer magnetizing, or just
fault interruptions, will affect the design. It is the responsibility of the end user of the circuit breaker to verify that
the breaker (ultimately the interrupter) is suited to meet the
specific needs of the system. The ability of the circuit breaker
to function properly on a given system will ultimately come
down to the review of the test data to ensure that the breaker
has been tested and verified to function correctly under the
duties in which it will be asked to perform.
Glenn A. Calhoon (1969), graduated with a B.S. Degree in Mechanical Engineering from Drexel University, Philadelphia PA in 1993. Glenn
joined Mitsubishi Electric Power Products Inc, in Warrendale PA in 2003.
His main responsibility is new product development for the Medium
Voltage Department of the Gas Circuit Breaker Division.
Kevin Beamer (1965), graduated with a B.S. Degree in Electrical
Engineering Technology from Pennsylvania State University, Harrisburg
PA in 1987. Kevin joined Mitsubishi Electric Power Products Inc, in
Warrendale PA in 1993. His main responsibility is development and
production for the Medium Voltage Department of the Gas Circuit
Breaker Division.
References
[1] Vacuum Switches and their Applications, Koichi
Iwahara, Denki Shoin, pp. 21-24, 1975. (in Japanese)
[2] Influence of Vacuum Arc Behavior on Current Interruption Limit of Spiral Contact, Toshinori Kimura,
Atsushi Sawada, Kenichi Koyama, Hiromi Koga, Tomotaka Yano
th
IEEE 19 Int. Symp. on Discharges and Electrical Insulation in Vacuum, 2000.
Satoshi Ochi (1974), graduated with a B.S. Degree in Material
Engineering from Ehime University, Japan, in 1997. He has been with
Mitsubishi Electric Corporation Power Distribution Systems Center
since 1997. His main field of development is vacuum interrupters for
medium voltage vacuum circuit breakers.
Kazuhiko Kagawa (1964), received a M.S. Degree in Naval Architecture from Osaka University, Japan, in 1989. He joined Mitsubishi Electric
Corporation, at Marugame Works in 1989. Since 1997, he has been with
Power Distribution Systems Center. His main field of development and
engineering is in vacuum circuit breaker technologies for medium voltage.
www.netaworld.org Summer 2010 NETA WORLD
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